1. Field of the Invention
The present invention relates to a conversion control circuit and a converter thereof; in particular, to a conversion control circuit with a built-in voltage-regulating circuit and a converter thereof.
2. Description of Related Art
Recently, because of the circuit simplicity, fewer components, and low cost, the non-isolated converter is widely used in the light emitting diode (LED) illumination markets.
FIG. 1 is a circuit diagram of the LED driving chip HV9910 generated by the American SUPERTEX company. A buck converter is shown in FIG. 1. The buck converter has a conversion control circuit 10a (i.e, the chip HV9910), an inductor L0, a diode D0, a power transistor Q0, and a current detection resistor R0. When the power transistor Q0 is turned on, the power provided by the voltage input terminal VIN may be supplied to the inductor L0 and an LED lamp string 20. When the power transistor Q0 is turned off, the power stored in the inductor L0 may be provided in form of current to the LED lamp string 20 which makes the LED lamp string 20 emitting light continuously. The conversion control circuit 10a controls the operation of the power transistor Q0 according to a feedback voltage signal Vcs from the current detection resistor R0, stabilizing the current flowing through the LED lamp string 20.
The conversion control circuit 10a directly connects to the voltage input terminal VIN for acquiring the requisite operating power. As shown in FIG. 1, the input voltage of the voltage input terminal VIN is converted into a power voltage VDD (which has the 7.5V power voltage) through the linear voltage-regulating circuit 11 of the conversion control circuit 10a, for providing the requisite working power to the conversion control circuit 10a. After the conversion control circuit 10a acquires enough power voltage VDD and starts to operate, the oscillator 12 then outputs a turn-on pulse to the input terminal S of the SR flip-flop 13, which makes the output terminal Q of the SR flip-flop 13 outputting a high voltage level signal turning on the external power transistor Q0.
When the power transistor Q0 is turned on, the current flows from the voltage input terminal VIN through the inductor L0, the LED lamp string 20, the power transistor Q0, and the current detection resistor R0 to the ground. Along with gradually increasing of current, as voltage level of the higher voltage end of the current detection resistor R0 reaches the reference voltage Vr0 (such as 250 mV), the comparator COMP0 outputs a high voltage level signal, to have the output terminal Q of the SR flip-flop 13 outputting a low voltage level signal turned off the power transistor Q0.
The inductor L0 may store the energy during the conducting period of the external-connected power transistor Q0, and may release the energy when the external power transistor Q0 is turned off. The released energy is a current which flows from the inductor L0 through the LED lamp string 20 and the diode D0 back to the inductor L0, until the oscillator 12 generates the next turn-on pulse to have the external power transistor Q0 turned on again. When the current flowing through the power transistor Q0 causing the voltage value of the higher voltage end of the resistor R0 reaching the reference voltage Vr0, the external power transistor Q0 may be turned off again, and the cyclic operations are repeated.
As described above, the LED driving chip HV9910 uses built-in linear voltage-regulation to regulate the voltage, and the power consumption during the processes may be represented as following functions:Power consumption(Pd)=(Vin−VDD)×IDD  (1)
Vin is the input voltage of the voltage input terminal VIN, VDD is power voltage, and IDD is the current for generating the power voltage VDD. According to the spec of the LED driving chip HV9910: VDD=7.5V, IDD=1 mA, and Vin=264×1.414=373V. The values are involved into the function (1), and the calculated power consumption (Pd)=(373−7.5)×1×10−3=0.37 (W).
According to the above calculation result, in application of high AC input voltage, the power consumption may be 0.37 W. To the LED lamp with 3 W output power, the ratio of power consumption is about 12.33% causing the converter to have low efficiency.
FIG. 2 shows a circuit diagram of the LED driving chip BP2808 generated by the Shang-Hai BPS company. As shown in FIG. 2, the conversion control circuit 10b (which is the chip BP2808) of the converter has an internal low-voltage transistor QL which is series-connected to the external-connected power transistor Q0. By controlling the operation of the internal low-voltage transistor QL of the conversion control circuit 10b, the operation of the external-connected power transistor Q0 may be synchronously controlled.
Different from the chip HV9910 which generates the turn-on pulse with constant frequency, the conversion control circuit 10b generates the turn-on pulse with constant off time. That is, when the cut-off time reaches a predetermined time length, the control unit 15 generates the turn-on pulse to turn on the low-voltage transistor QL, pulling down the voltage level of the source node of the power transistor Q0, for making the power transistor Q0 turn on. At the moment, the current starts to flow from the voltage input terminal VIN through the inductor L0, the LED lamp string 20, the power transistor Q0, the low-voltage transistor QL, and the current detection resistor R0 to the ground. When the current makes the voltage level of the higher voltage end of the current detection resistor R0 increase to the reference voltage, the control unit 15 then turns off the internal low-voltage transistor QL and the external-connected power transistor Q0. The cyclic operations are repeated.
The converter uses a voltage-regulating diode Z0 for converting the power transmitted from the voltage input terminal VIN into the power voltage VDD for providing the requisite operating power to the conversion control circuit 10b. The present converter is different from the chip HV9910 which uses the linear voltage-regulating circuit 11 within the conversion control circuit 10a for converting the input voltage of the voltage input terminal VIN into the power voltage VDD.
Thus, the chip BP2808 uses external components which are combined into a linear voltage regulator. The power consumption during the processes may be represented as following functions:Power consumption(Pd)=(Vin−VLED−VDD)×(IDD+IZK)  (2)
Vin is the input voltage of the voltage input terminal VIN, VLED is the voltage drop of the LED lamp string 20, VDD is the power voltage, IDD is the current for generating the power voltage VDD, and IZK is the current flowing through the voltage-regulating diode Z0. According to the spec of the chip BP2808: VDD=12V, IDD=0.2 mA, Vin=264×1.414=373V, VLED=10V, and IZK=1A. The values are substituted into the function (2) for calculating the power consumption (Pd)=(373−10−12)×1.2×10−3=0.42 (W).
FIG. 3 shows a circuit diagram of the LED driving chip GR8210 generated by the Taiwan GRENERGY company. Excepting for the voltage-regulating manner is different from the chip BP2808, the remaining operations of the chip GR8210 are essentially the same as those of the chip BP2808.
The presented chip uses the internal low voltage linear voltage-regulation method regulating the power, instead of using external-connected linear voltage regulator. As shown in FIG. 3, the conversion control circuit 10c has a built-in low voltage linear voltage-regulating circuit 14. One end of the linear voltage-regulating circuit 14 is connected to the source of the power transistor Q0 while the other end thereof is connected to an external-connected capacitor C0, for generating the power voltage VDD. When the control unit 15 controls the internal low-voltage transistor QL to turn off, the low voltage linear voltage-regulating circuit 14 may generate a charging current for charging the external capacitor C0. At the moment, the power transistor Q0 is at the status of semi-conducting and has high resistance property. That is, the power transistor Q0 provides the requisite operating current to the low voltage linear voltage-regulating circuit 14 for generating the power voltage VDD by suffering the high voltage status.
The chip GR8210 uses built-in low voltage linear voltage-regulating for regulating voltages, and the power consumption during the processes may be represented by the following functions:Power consumption(Pd)=(Vin +VD−VDD)×IDD  (3)
Vin is the input voltage of the voltage input terminal VIN, VD is the voltage drop of the diode, VDD is the power voltage, and the IDD is the current for generating power voltage VDD. According to the spec of the chip GR8210: VDD=5V, IDD=0.9 mA, and Vin=264×1.414=373V. The values are substituted into the function (3) for calculating the power consumption (Pd)=(373+0.7−5)×(0.9)×10−3=0.33 (W).
On the basis of the above, because the high voltage linear voltage regulator is built-in, the linear conduction loss of the LED driving chip HV9910 during the processes of voltage-regulating is directly generated within the chip, which may easily cause temperature increasing. The LED driving chip BP2808 uses external resistor and voltage-regulating diode which are combined into a linear voltage regulator for carrying a great part of linear conduction loss, which may reduce the temperature of the controller. However, in the aspect of overall efficiency of the power voltage-regulating, the problem of high power consumption may not be improved regardless whether the voltage regulator is built-in or external-connected.